The purpose of the proposed research project is: (1) to obtain information about the structures and catalytic mechanisms of dihydrofolate reductases and thymidylate synthetases from various sources (with particular reference to the inhibition of these enzymes by amethopterin and fluorodeoxyuridylate), and to complete similar mechanistic studies on other folate coenzyme-dependent enzyme systems such as methionine synthetase, folinate isomerase and serine hydroxymethylase; (2) to obtain information about the mechanisms by which folate compounds, particularly folate, 5-methyl tetrahydrofolate and amethopterin, are transported into cells; and (3) to utilize the above information in the design of new cancer chemotherapeutic regimens.